We're working on an official timeline for our story bible, but the main thing to keep in mind is that the Toa are only brought into being to deal with the Great Beings, therefore there is no friendly interaction between the Toa and either Makuta or Ekimu.
I think that's a great way to do it. Personally, I feel like the Barraki fit the best as their older warlord selves, and it can lend itself to better flesh out the Bionicle G3 backstory, as well as paint a backdrop for web serials or the like to expound on.
Full disclosure: I feel like Bionicle G1's main downfall was not so much that it was very complicated, but that the main story was not conveyed properly to enough people. The serials told side stories, but they ended up becoming a decent part of the main plot. For me, the web original series were best when they told a story of the past, such as The Many Deaths of Toa Tuyet and Birth of a Dark Hunter.
So a story focusing on some of the previous Toa and the Barraki would be fun to see, but something that would be given as extraneous information later on into the years.
We toyed around with a concept kind of like Mahri Nui, a village where the Nahoans exclusively used the Mask of Water Breathing until the last Gali took them away. We never settled on that, but it's not completely outside the purview of the cast.
Not sure where we stand on this "Colossi" concept. Hydraxon and his role might be reserved for something else entirely.
It would not be. The Elemental Gods were brought into the world after the Matoran, not the other way around. Makuta is mad at Ekimu for bringing life into the island, that life is the Matoran, so if he had made the Elemental Gods first, there wouldn't be any Matoran because that's where they would have drawn the line at creating sentient life.
Karzahni is sealed away before the Matoran or the Elemental Gods are created, which is why neither Ekimu or Makuta consider him a possibility when the Lightning Tribe appears.
It's worth noting that Karzahni, while possibly being hinted at, will be an unknown for the entirety of the three year pitch.